This application relates to an improved method and apparatus for testing and/or adjusting vibration absorbers for suspended cables, i.e., devices for absorbing energy to suppress mechanical subspan oscillation and aeolian vibration of such cables.
The testing arrangement herein described is particularly suitable for, but not limited to, testing of vibration absorbers operating on the principles disclosed in copending U.S. patent application Ser. No. 147,096, filed May 7, 1980, now U.S. Pat. No. 4,346,255, assigned to the assignee of the present application and entitled "Overhead Electrical Conductor System Including Subspan Oscillation And Aeolian Vibration Absorber For Single And Bundle Conductors". This prior application discloses and claims an essentially dissipative (as opposed to spring-type) vibration absorber, having a damping mechanical impedance which essentially matches that of the transmission line to which the damper is attached. The acceptacle range of damping impedance of the absorber is indicated as being anywhere between half and twice the transmission line mechanical impedance; although we have found that satisfactory performance may be obtained with a damping impedance as high as three times the transmission line mechanical impedance.
By essentially matching the transmission line mechanical impedance, and providing a dissipative (i.e., frequency independent) damping effect, the vibration absorber of the aforesaid U.S. Pat. No. 4,346,255 provides optimum energy coupling between the transmission line and absorber, thus effectively absorbing travelling waves on the line before they build up to large amplitude standing waves which can cause damage to the line and associated supporting elements.
In order to test such vibration absorbers (as well as those of the type described, e.g., in U.S. Pat. No. 3,885,086), and to adjust them to the proper damping value, a full scale test line comprising a span of suspended cable is normally required. Such a test arrangement is necessarily complicated and costly.
One such testing method consists of exciting a suspended cable span from a variable frequency force generator and measuring the displacement of the cable at the standing wave maxima and minima over a band of resonant frequencies, with and without the vibration absorber in place. Another variation of this method is to measure the power input to the cable span at the resonant frequencies and at specified cable displacements with and without the vibration absorber in place. Since both the cable displacements and the power input to the span depend on the location of the force generator and the vibration control device within the span, the above measurements give only a relative measure of absorber effectiveness and not the actual damping value. Consequently, the results are of very limited use. For example, it would not be possible to adjust the damping of a vibration absorber to a desired value by this method.
Another method of comparing different vibration absorbers consists of displacing the cable in the test span, releasing and allowing it to oscillate. From the record of the resultant decaying oscillation the term known as the logarithmic decrement .sigma. is calculated. This decrement is obtained by taking the natural logarithm of the ratio of successive displacement amplitudes in time, i.e., amplitudes which are one cycle apart. When these measurements are made with and without the vibration absorber, a relative measure of their effectiveness can be established. However, this type of test can only be carried out at the fundamental and perhaps at the second harmonic frequency of the span. The measured displacements depend on the vibration absorber location and on the location of the measurement transducer. Since the results do not give the numerical value of the damping, this method is not useful in adjusting the damping to a particular value.
Accordingly, an object of the present invention is to provide a suspended cable vibration absorber testing and/or adjusting arrangement which is relatively simple and which can be carried out in an indoor testing facility and in a relatively small space.